Two-piece drawn and ironed metal containers, such as those used to contain carbonated beverages, have an integrated body/bottom piece and a separate top piece. The body includes a cylindrical sidewall section and a bottom support section upon which the container rests when upright. A typical pressurized aluminum beverage container also has a transition or rework taper section which connects the sidewall section, having one diameter, with the bottom section, having a smaller diameter.
A drawn and ironed container body is formed on a body maker which typically comprises a reciprocating punch, a redraw sleeve, a redraw die, one or more ironing rings, a compound doming die, and an air stripper. The nose of the punch engages the open end of a shallow, cup-like container blank. The cup is forced linearly by the punch through the redraw die to reduce its diameter and elongate the sidewalls. The punch then forces the redrawn cup through the series of ironing rings to gradually thin and further elongate the sidewalls. The punch then forces the bottom of the container against the compound doming die to form an inward dome and an annular support in the bottom section of the container. The punch and the completed container body reverse direction and the container is separated from the punch by a burst of compressed air, sometimes with the assistance of stripping fingers. The container body is then transported to be necked and flanged.
As the nose of the punch forces a container cup through the redraw die and ironing rings to thin and elongate the sidewall, metal is pulled around the nose of the punch along the side of the punch, leaving a slight annular, outward bulge where the metal bends in the rework taper section between the sidewall and bottom sections. The bulge can expand after the doming operation when compressed air is injected into the container to facilitate its removal from the punch. As will be discussed below, the bulge (sometimes known as the "stripper bulge") is thought to introduce weakness in the container.
The nose of the punch has a slight taper, known as the rework taper, having dimensions which define the thickness of the container wall in the rework taper section. This thickness has a significant influence on the axial load capacity, or column strength, of the completed container. The column strength of a container is a measure of the container's ability to resist compressive forces applied between the top and bottom of the container in a direction substantially parallel to the sidewall. To determine the column strength of a container body and to determine where it first fails when an excessive axial load is applied, the container body (without the top) is placed in an axial load tester with one end against a base plate. When the machine is turned on, the base plate and a second, parallel plate move toward each other, thus compressing the container body longitudinally between the two plates. The compressive force is automatically increased until the container fails, such as by collapsing. The force on the container at the time of failure is the column strength of the container and is read from a display on the machine.
Fillers and distributors, to whom empty, necked containers are sold, currently require that the empty containers have a column strength of at least 250 pounds. A container with insufficient column strength may fail by collapsing when subjected to normal axial loads of up to about 250 pounds, as might occur when containers are necked, filled or seamed. In the past, gauges of metal, such as aluminum alloy with a thickness of 0.0120 inches (12.0 mils) or greater were employed and the strength requirements were readily attainable.
However, in order to reduce the amount of metal used in a container, and to thus reduce production costs and consumption of energy and raw materials, containers have been downgauged in recent years, such as to 11.8 mils and, more recently, to 11.6 mils. For the same reasons, still further downgauging is desired. Accompanying the downgauging has been a reduction in column strength and an increase in the number of container failures. Viewed statistically, both the average column strength and the minimum column strength of a sampling of aluminum alloy container bodies have declined to such an extent that, as set forth in Table 1, the minimum column strength is less than 250 pounds: not all containers in the sampling met the 250 pound requirement. A lack of uniformity in the containers in the sampling is demonstrated by the high standard deviation noted in Table 1.
TABLE 1 ______________________________________ NECKED COLUMN STRENGTH (11.6 mil gauge aluminum alloy) ______________________________________ AVERAGE 306 pounds MINIMUM 202 pounds MAXIMUM 338 pounds STANDARD DEVIATION 30 ______________________________________
It can be expected that further downgauging will result in further undesirable decreases in the minimum and average column strength and an increase in the standard deviation unless offsetting measures are taken.
In addressing this problem, the present inventors have now recognized a possible relationship between the column strength of a container and the presence of surface defects and the stripper bulge. It is believed that minor defects which can develop in the surface during container processing, handling and shipping, such as small bumps and dents in the sidewall and elsewhere, can reduce the column strength of a container. It is also believed that the presence of the stripper bulge in the rework taper section can also contribute to reduced column strength. The present inventors believe that the thicker gauges of containers produced in the past (such as 12 mils or greater) have enabled the containers to better able to resist the formation of defects and the stripper bulge than one with a thinner gauge, and to better resist the effects of any defects or the stripper bulge which occurred.
As a result of the foregoing factors, a significant number of containers with thinner gauges than previously employed may fail to meet minimum column strength requirements, even though the average column strength of the containers in a sampling exceeds the requirement. It is, therefore, desirable to increase the uniformity of the containers in a sampling and to increase the minimum column strength of containers by decreasing the effects of the stripper bulge and surface defects and thereby enable the column strength requirement to be met by substantially all of the containers. It is also desirable to be able to meet the column strength requirement when further downgauging of the container body thickness is undertaken.